* Agency proposes rule to spell out bag, seat charges
* Move comes as industry backs bill to play up base fares
(Adds comment from consultant, background, byline)
May 21 (Reuters) - Airlines should disclose fees for checked baggage, carry-on items and other à la carte services to make it easier for travelers to discern the true cost of a ticket, the U.S. Department of Transportation said on Wednesday.
The new disclosure rules, which build on regulations from 2009 and 2011, would also stipulate that online flight search sites such as Expedia Inc, Google Inc and Kayak provide more information.
U.S. carriers would have to spell out specific charges for carry-on items and advance seat assignments.
The proposed additional consumer protections come as the airline industry supports legislation that would roll back rules requiring greater disclosure of fees and taxes tied to airfares.
A bill called the Transparent Airfares Act of 2014 would allow airlines to play up base fares in advertising, but disclose taxes and fees separately. The measure is backed by Airlines for America, a Washington-based lobby for U.S. airlines. A counter measure to keep the current rules in place was recently introduced by a U.S. senator.
The DOT said it expects to issue a final rule tied to the latest proposals next year.
The U.S. government move is "a direct pushback to what A4A (Airlines for America) is trying to do, which is to really make things more opaque," said Robert Mann, an airline consultant based in Port Washington, New York.
"That's the way airlines used to advertise and the DOT banned it long ago," Mann said.
In recent years, airlines have rolled out extra charges for items that were once included in the price of an airline ticket such as baggage fees. More airlines have been piling on these so-called ancillary fees, with low-cost carrier Frontier Airlines last month disclosing it would cut base fares, but implement new charges for carry-on bags and certain seats.
In its statement, the Transportation Department said charges for additional air-travel services were difficult for buyers to determine up front.
The proposal would also require airlines and agents to disclose on website itinerary displays whether flights sold are operated in partnership with other carriers. Big travel agents would have to respond quickly to consumer complaints and offer the option of holding reservations at quoted fares without payment for 24 hours if made a week or more before departure.
The agency is also looking to expand the number of carriers reporting on-time performance and mishandled baggage rates to airlines that account for at least 0.5 percent of U.S. scheduled passenger revenue from 1 percent. The DOT said that change would affect carriers such as discounter Spirit Airlines, which charges fees for many other services.
Airlines for America did not immediately comment on the transportation agency's proposed rules. A Southwest Airlines spokesman said the carrier had not had enough time to review the proposals.
(Reporting by Karen Jacobs in Atlanta; editing by G Crosse and Paul Simao)